One of the most annoying - and common - things on eBay is the number of sellers who try to pull a fast one on you by listing an item at a cheap price, but charging a stupidly high postage & packing charge. Excessive postage charges are specifically prohibited by eBay rules (see http://pages.ebay.co.uk/help/policies/listing-shipping.html for details), but a lot of sellers either are not aware of this, or simply don't care.
Unfortunately, at present it is not possible to take postage charges into account when filtering your browsing results by price, even though unless an item has a "collect in person" option, you have no choice but to add them on to find your final bill. A lot of your search results are therefore likely to show items that in fact are well beyond the budget you have specified, and that being so, I've written this quick guide, which I hope will be useful.
The link to eBay's help page above gives £25 for a DVD for standard delivery as an example of an excessive postage charge, but there's little detail beyond that as to what is fair and what is not. I'm not covering very specialised iterms here, some of which genuinely do require very careful and expensive handling, but those items we all see every day on the site. My personal recommendations would be:
* Be especially alert when buying electronic and photographic components: excessive postage charges are near-epidemic in some categories in these areas.
* Remember that a reasonable charge over and above the actual cost of the stamps is perfectly acceptable: it's postage and packing, after all. A couple of pounds for a CD might be more than some other sellers charge, but it's not unfair. Ten pounds for a CD is likely to be unfair.
* Avoid auctions for small items using Buy It Now that are listed at (eg) 99p, but with £10 postage charges: either the seller is trying to get around paying appropriate listing fees (which is also prohibited by eBay), or they simply haven't bothered to work out a fair cost. Either way, steer clear.
* Avoid sellers advertising attractive opening prices, but shipping from distant locations such as Hong Kong or Singapore, as you can easily find yourself paying £30 for the "privilege" of getting a fairly small item shipped to you. As a comparison, from a UK seller, even an entire PC should cost less in p&p than that!
* At the bottom of the page I linked to above is a "Report" link, with which you can alert eBay to sellers charging excessive postage. Use it, even if the problem you find is in an auctions you're not personally interested in. I'd recommend only using this link for the most blatant examples, though, as you can only report ten listings at one time.
* Avoid sellers who charge a sizeable compulsory "insurance" fee, over and above the usual charges for Recorded Delivery, Special Delivery etc, unless there's a very good justification indeed for it. Charging several pounds for insurance on an item which is to be sent by standard First/Second Class post is not fair, and you shouldn't accept it.
* Use the "Ask seller a question" feature to query why their p&p charges are so high. If they don't give a satisfactory answer, or worse still give no answer at all, walk away.
* Don't take "if you don't like these terms, don't bid" for an answer; as with that earlier scourge, PayPal surcharges, a comment like that is absolutely no defence against something specifically prohibited by eBay policy. Unfair terms are the seller's problem, not your problem as the buyer
* Most of all, don't bid on auctions with unfair postage terms in the first place. The only thing guaranteed to stop sellers trying this one on is if interest in their listings dries up as a result.