You would be able to claim the VAT on stock back, but if will be covering the VAT your customers are paying then you will be worse off.
For example you buy £1000 worth of stock and get charged VAT at 20% so you pay £1200.
You then sell this with a 50% markup to your customers, so £1500 but you have to also include VAT in that so you actually charge them £1250 + £250 VAT making the sale price the same £1500.
Whilst you can then recover the £200 VAT you paid, you have to pay the HMRC £250 as thats the VAT you have collected, leaving you £50 out of pocket.
The only way it would work in your favour is if you can charge your customers the £1500 + VAT as they would then pay £1800. You would collect £300 in VAT offset that against the £200 you paid and leave you having to pay the HMRC £100.
But your customers would be £300 worse off.